The amount of trans fat in the American bloodstream fell by more than half after the Food and Drug Administration required food manufacturers to label how much of the unhealthful ingredient is in their products, according to a new study.

Blood levels of trans fat declined 58 percent from 2000 to 2008. FDA began requiring trans-fat labeling in 2003. During the same period several parts of the country, New York most famously, passed regulations limiting trans fats in restaurant food and cooking. The makers of processed food also voluntarily replaced trans fats with less harmful oils.

The decline, unusually big and abrupt, strongly suggests government regulation was effective in altering a risk factor for heart disease for a broad swath of the population.

Researchers at the Centers for Disease Control and Prevention discovered the decline by analyzing blood drawn as part of the National Health and Nutrition Examination Survey, which interviews and examines a sample of Americans at least once a decade.

The trend was seen in white adults; researchers are looking to see if it occurred in other ethnic and racial groups too.

Trans fats, which are used for deep-frying and as an ingredient in baked goods and spreads, increase the risk of heart disease. One study found that if a person increases total calorie intake 2 percent all in the form of trans fat, risk of a heart attack rises by about 20 percent.

“Our findings provide information about the effectiveness of these interventions,” said Hubert Vesper, a CDC chemist who headed the analysis. “This reduction is substantial progress that should lower the risk of cardiovascular disease in people.”

The study, which appears in the Journal of the American Medical Association, also found a decrease in LDL (the so-called “bad cholesterol”) and an increase in HDL (the “good cholesterol) between 2000 and 2009. That healthful trend could be a consequence of the trans-fats decline, other dietary changes, increase in exercise, or use of cholesterol-lowering statin drugs.

New York City’s trans-fat ban took effect in December 2006. It covered only “artificial” trans fats created in the manufacturing of cooking oil and other products. It didn’t restrict trans fat that naturally occurs in some foods.

By November 2008, 98 percent of restaurants were not using trans fats in oils, shortenings and spreads, compared with 50 percent in the year before the ban.

California enacted a law banning trans-fat in restaurant food by 2010, and in retail baked goods by 2011. Other jurisdictions have enacted bans as well.